1. Locate the folder of this mod within the Steam Workshop folder: ...\steamapps\workshop\content\270880\759043939\
2. Enter the folder of the content for the current game version: 138_content (this is used since ATS v1.38, that's why it's called so).
3. Archive the content of this folder (the material and ui folders and the files in the root) into a ZIP file inside the game's mod folder at c:\Users\[yourname]\Documents\American Truck Simulator\mod\
4. Give the ZIP a name you like. I named it Speed limit sign in Route Advisor.zip. Directly inside the ZIP, you should have the above mentioned 2 folders and 3 files, but no common folder (like the original 138_content).
5. Rename the zip extension to scs: Speed limit sign in Route Advisor.scs
6. Unsubscribe from this mod (it seems to be abandoned anyways). This will delete the 759043939 folder in some time.
7. In the game's Mod Manager, place your new mod in your load order into where the this Steam mod was (now displayed with the red exclamation point, since you unsubbed from it).
8. Check if it works.